Beren and Luthien is about the fate of two characters — a mortal man and an immortal elf — taken from Tolkien's fictional world, Middle Earth.
The story centres on a series of daunting quests and forbidden love, as the lovers together try to steal from the greatest of all evil beings, Melkor.
It was edited by his son Christopher Tolkien, who is now 92, and contains versions of a tale that became part of The Silmarillion.
Written after the British author came back from France in 1916, the book served as an "exorcism" of the appalling experiences he had on the battlefields of WWI, Tolkien scholar John Garth said.
"When he came back from the trenches, with trench fever, he spent the winter [of 1916-1917] convalescing," Mr Garth told the BBC.
"He'd lost two of his dearest friends on the Somme and you can imagine he must have been inside as much of a wreck as he was physically."
The book's illustrations are by Alan Lee, who won an Academy Award for work on Peter Jackson's film adaption of the Lord of the Rings trilogy.
Those books have sold over 150 million copies worldwide, while the Lord of the Rings films have grossed over $1 billion altogether.
Tolkien died in 1973 at age 81.
The names Beren and Lúthien are carved on the gravestone Tolkien and his wife share in Wolvercote cemetery in Oxford.